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151.
Bor-ming Jahn Ramon Capdevila Dunyi Liu Antoine Vernon G. Badarch 《Journal of Asian Earth Sciences》2004,23(5):629
The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust in the Phanerozoic. Mongolia is the heartland of the CAOB and it has been subject to numerous investigations, particularly in metallogenesis and tectonic evolution. We present new petrographic, geochemical and Sr–Nd isotopic analyses on Phanerozoic granitoids emplaced in west-central Mongolia. The data are used to delineate their source characteristics and to discuss implications for the Phanerozoic crustal growth in Central Asia. Our samples come from a transect from Bayanhongor to Ulaan Baatar, including three tectonic units: the Baydrag cratonic block (late Archean to middle Proterozoic), the Eo-Cambrian Bayanhongor ophiolite complex and the Hangay–Hentey Basin of controversial origin. The intrusive granitoids have ages ranging from ca. 540 to 120 Ma. The majority of the samples are slightly peraluminous and can be classified as granite (s.s.), including monzogranite, syenogranite and alkali feldspar granite. Most of the rocks have initial 87Sr/86Sr ratios between 0.705 and 0.707. Late Paleozoic to Mesozoic granitoids (≤250 Ma) are characterized by near-zero Nd(T) values (0 to −2), whereas older granitoids show lower Nd(T) values (−1.5 to −7). The data confirm the earlier observation of Kovalenko et al. [Geochemistry International 34 (1996) 628] who showed that granitoids emplaced outside of the Pre-Riphean basement rocks are characterized by juvenile positive Nd(T) values, whereas those within the Pre-Riphean domain and the Baydrag cratonic block, as for the present case, show a significant effect of ‘contamination’ by Precambrian basement rocks. Nevertheless, mass balance calculation suggests that the granitoids were derived from sources composed of at least 80% juvenile mantle-derived component. Despite our small set of new data, the present study reinforces the general scenario of massive juvenile crust production in the CAOB with limited influence of old microcontinents in the genesis of Phanerozoic granitoids. 相似文献
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153.
Intercomparison of Boron Isotope and Concentration Measurements. Part II: Evaluation of Results 总被引:3,自引:1,他引:3
Roberto Gonfiantini Sonia Tonarini Manfred Grning Alessandra Adorni‐Braccesi Assad S. Al‐Ammar Marcus Astner Sebastien Bchler Ramon M. Barnes Randy L. Bassett Alain Cocherie Annette Deyhle Andrea Dini Giorgio Ferrara Jrme Gaillardet Judith Grimm Catherine Guerrot Urs Krhenbühl Graham Layne Damien Lemarchand Anette Meixner D. Jack Northington Maddalena Pennisi Eva Reitznerov Ilia Rodushkin Naoji Sugiura Regina Surberg Sabine Tonn Michael Wiedenbeck Samuel Wunderli Yingkai Xiao Thomas Zack 《Geostandards and Geoanalytical Research》2003,27(1):41-57
The Istituto di Geoscienze e Georisorse (IGG), on behalf and with the support of the International Atomic Energy Agency (IAEA), prepared eight geological materials (three natural waters and five rocks and minerals), intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials were distributed to twenty seven laboratories - virtually all those performing geochemical boron isotope analyses in the world -which agreed to participate in the intercomparison exercise. Only fifteen laboratories, however, ultimately submitted the isotopic and/or concentration results they obtained on the intercomparison materials. The results demonstrate that interlaboratory reproducibility is not well reflected by the precision values reported by the individual laboratories and this observation holds true for both boron concentration and isotopic composition. The reasons for the discrepancies include fractionations due to the chemical matrix of materials, relative shift of the zero position on the δ11 B scale and a lack of well characterized materials for calibrating absolute boron content measurements. The intercomparison materials are now available at the IAEA (solid materials) and IGG (waters) for future distribution. 相似文献
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155.
Dr. Ramon Vegas 《International Journal of Earth Sciences》1975,64(1):266-278
Mapping of the Southwestern Hesperian Massif (South Western Spain) has permitted the definition of two principal systems of wrench faults in conjugate directions, SW-NE and NNW-SSE. By their dimensions and their association with deep basic rocks, they may be considered as accidents which interest the entirety of the crust. The directions of these systems are enclosed in a scheme of late-Hercynian wrench faults produced in the dismantling of the Iberian Peninsula at the end of the orogenic Hercynian phases. According to these large lines of fracture it is possible to relate a) the areas of Mesozoic sedimentation, pre-Alpine distension, b) the reliefs of the Mesozoic covering of the interior of the Hesperic Massif, c) the reliefs and the tectonic basins of the interior of the Meseta and d) the possible evolution of the Iberian Plate with relation to the Bay of Biscay and the areas of Alpine compression of the meridional edge, Betic Chains. 相似文献
156.
157.
The transfer of sediment through a highly regulated large fluvial system (lower Ebro River) was analysed during two consecutive floods by means of sediment sampling. Suspended sediment and bedload transport were measured upstream and downstream of large reservoirs. The dams substantially altered flood timing, particularly the peaks, which were advanced downstream from the dams for flood control purposes. The suspended sediment yield upstream from the dams was 1 700 000 tonnes, which represented nearly 99 per cent of the total solid yield. The mean concentrations were close to 0·5 g l?1. The sediment yield downstream from the dams was an order of magnitude lower (173 000 tonnes), showing a mean concentration of 0·05 g l?1. The dams captured up to 95 per cent of the fine sediment carried in suspension in the river channel, preventing it from reaching the lowermost reaches of the river and the delta plain. Total bedload transport upstream from the dams was estimated to be about 25 000 tonnes, only 1·5 per cent of the total load. The median bedload rate was 100 gms?1. Below the dams, the river carried 178 000 tonnes, around 51 per cent of the total load, at a mean rate of 250 g ms?1. The results of sediment transport upstream and downstream from the large dams illustrate the magnitude of the sediment deficit in the lower Ebro River. The river mobilized a total of 350 000 tonnes in the downstream reaches, which were not replaced by sediment from upstream. Therefore, sediment was necessarily entrained from the riverbed and channel banks, causing a mean incision of 33 mm over the 27 km long study reach, altogether a significant step towards the long‐term degradation of the lower Ebro River. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
158.
Abderrahim Essaifi Jean-Louis Lagarde Ramon Capdevila 《Comptes Rendus Geoscience》2006,338(5):386-388
159.
Jennifer?M.?GarrisonEmail authorView authors OrcID profile Kenneth?W.?W.?Sims Gene?M.?Yogodzinski Ricardo?D.?Escobar Sean?Scott Patricia?Mothes Minard?L.?Hall Patricio?Ramon 《Contributions to Mineralogy and Petrology》2018,173(1):6
Sumaco Volcano is located in the rear-arc of Ecuador and produces phonolitic alkaline lavas hosting a unique assemblage of minerals including haüyne and titanaugite. The most mafic lavas are picrobasalts that contain titanaugite as the primary mineral phase; the most evolved tephri-phonolite lavas contain titanaugite?+?anorthoclase?+?haüyne. Titanaugite forms at middle to deep crustal pressures, whereas haüyne is only stable at shallow depths in highly oxidizing conditions. The Sumaco mineral assemblages and geochemistry indicate that fractionation of the titanaugite- and haüyne-bearing assemblage took place over a range of pressures from 5 to 25 kbar (14–75 km), with at least 50% of differentiation taking place at shallow crustal levels. Minerals record multiple cycles of recharge and mixing accompanied by an increase in fO2 and sulfur concentration during differentiation. Mantle-like Sr and Nd isotope values (87Sr/86Sr = 0.70406–0.70423; 143Nd/144Nd = 0.512880–0.512913) indicate minimal crustal assimilation. Sumaco’s unique geochemical composition is not observed in the nearby volcanoes Antisana, Pan de Azucar or El Reventador suggesting that its unique magma source is confined to this volcano. The high temperature and sulfate-saturated conditions at shallow depths suggest that magma ascends rapidly to a shallow reservoir where the majority of crystallization and recharge takes place prior to eruption. An important conclusion of this research is that Sumaco does not represent typical rear-arc subduction processes, and caution should be used when using Sumaco as an end-member to evaluate across-arc processes in the Northern Volcanic Zone. 相似文献
160.